This invention relates to submersible propulsor units, and is specifically concerned with an improved integral motor propulsor unit for water vehicles that provides high thrust, low weight, low noise, and easy maintenance.
Electric motor type propulsor units for water vehicles are known in the prior art. While such propulsors may be used for surface vessels, they find their primary application as secondary drive units for submarines where reliability, control, and low noise emissions are at a premium. In the prior art, such propulsor units have typically comprised a "canned" or wet winding electric motor having an output shaft that is connected to a propeller. The motor is disposed either directly in front of or behind the propeller. Unfortunately, the fact that the "canned" motor is disposed either directly in front of or behind the flow of water generated by the propeller creates obstructions to fluid flow that tends to reduce the effective thrust that can be generated by these units. To reduce the thrust losses caused by this blockage, higher speed and smaller diameter motors were used. However, the high shaft speed results in high propeller cavitation, which in turn generates a high level of unwanted noise.
To overcome these shortcomings, the Westinghouse Electric Corporation developed an integral motor propulsor unit that is disclosed and claimed in U.S. Pat. No. 4,831,297. This particular propulsor unit resembles a jet engine in structure and generally comprises a cylindrical shroud having a water inlet and a water outlet, a propeller having a hub rotatably mounted within the shroud on a shaft that is concentrically mounted within the shroud by a plurality of support vanes, and an electric motor for driving the propeller that includes an annular rotor mounted around the periphery of the propeller blades, and a stator that is integrated within the shroud of the unit. The advanced design of this particular prior art propulsor unit substantially increases the thrust output for a propulsor for a given weight and size while at the same time reducing the amount of noise generated due to the largely unencumbered flow of water that the propeller of the device can force through the fluid-dynamically shaped shroud. The quietness of the unit is further improved due to the noise-blocking characteristics of the shroud.
While the aforementioned integral motor propulsor unit represents a substantial advance in the art, the applicants have noted a number of areas in the design of this device which could bear improvement. For example, the water lubricated thrust and radial bearings periodically need to be replaced. To do this, the unit must be dry-docked. Additionally, if any of the components of the bearings need to be inspected or replaced, the location of these bearings necessitates an almost complete disassembly of the propulsor unit. The applicants have noted that the support vanes located upstream of the propeller can induce cavitation in the water surrounding the propeller during the operation of the propulsor, which in turn not only creates unwanted noise, but further impairs the efficiency of the unit. The applicants have also observed that the induction-type motor arrangement used in this particular prior art propulsor unit necessitates a very close spacing between the outer diameter of the rotor and the inner diameter of the stator if the electromagnetic coupling between the rotor and the stator is to be effectively implemented. However, such close spacing not only creates drag forces from the thin film of water that is disposed between the stator and the rotor; it also generates additional unwanted noises by increasing the magnitude of the harmonic currents flowing through the rotor (which are always present to some degree due to dissymmetries in the magnetic fields generated by the stator), which in turn cause the rotor to vibrate. The close spacing required between the inner diameter of the stator and the outer diameter of the rotor also creates an unwanted area of vulnerability in the propulsor unit should it be subjected to a high level of mechanical shock, or should sea water debris collect between the stator and rotor.
Clearly, there is a need for an improved, integral motor-type propulsor unit for use in submarines or other water vessels that has a bearing assembly which does not necessitate dry-dock procedures and which is relatively simple and easy to perform an inspection or a maintenance operation on. Ideally, such a propulsor unit would have lower noise characteristics than prior art units, and would incorporate a design which does not necessitate such close spacing between the rotor and stator in order to decrease the vulnerability of the unit in this region to mechanical shock or the collection of sea water debris.